The present invention relates to the construction of grating and other structures in photosensitive waveguides such as optical fibre devices.
The creation of a grating in a photosensitive waveguide material utilising the interference pattern from two interfering coherent UV beams is well known. This technique for construction of Bragg gratings is fully described in U.S. Pat. No. 4,725,110 issued to W H Glenn et. al. and U.S. Pat. No. 4,807,950 issued to W H Glenn et. al.
Bragg grating structures have become increasingly useful and the demand for longer and longer grating structures having higher and higher quality properties has lead to the general need to create improved grating structures.
PCT patent application No. PCT/AU96/00782 by Ouellette et. al. discloses an improved low noise sensitivity interferometric arrangement which operates on a xe2x80x9cSagnac loopxe2x80x9d type arrangement.
Australian Provisional Patent Application No. PP3816 by Stepanov et. al., assigned to the present applicant, discloses an advanced form of interferometric writing system utilising a modulator to control the position of an interference pattern on a fibre.
Unfortunately, there is an ever present need for providing further improved grating structures.
It is an object of the present invention to provide for an improved grating writing system.
In accordance with a first aspect of the present invention, there is provided a method of forming a complete grating structure on a photosensitive waveguide comprising the steps of: (a) writing an initial portion of the grating structure on the waveguide; (b) testing optical properties of the initial portion to determine a series of parameters of the initial portion; (c) utilizing the parameters as a feedback to alter the characteristics of a subsequently written portion of the grating structure to provide for an improved form of grating structure; (d) iterating the steps (a) to (c) so as to form the complete grating structure.
The writing can be performed utilizing a coherence pattern formed from the interference of two coherent beams on the waveguide. The characteristics can include the intensity or phase of the subsequently written portion. The testing may include determining the spectral reflectance transmittance, or group delay response of the initial portion and of subsequently written portions.
The step (a) further can comprise the step of locally perturbing the intensity, phase or frequency of the grating structure and the step (b) can comprise measuring the local perturbation. The local perturbation can comprise a periodic modulation and the testing step further can comprise measuring sum and difference frequencies for the modulation.
The testing can comprise averaging portions of the reflectance spectrum, transmittance spectrum and group delay spectrum of the initial portion and of subsequently written portions.